In general, a balun, which is an abbreviation of “balance-unbalance,” is a device that transforms unbalanced or single-ended signals to balanced or differential signals, or vice versa. A balun can categorized as either an active balun or a passive balun, depending on whether the balun includes active devices or not.
A conventional passive balun is generally a transformer 1, as illustrated in FIG. 1. Referring to the transformer of FIG. 1, when a single-ended signal is applied to the primary winding, magnetically induced current from the primary winding flows through the secondary winding. Differential signals are obtained from the ends of the secondary winding. A passive balun can operate irrespective of the amplitude of the input signal. Further, the phase and amplitude balance errors for a passive balun are smaller than those of an active balun, and the operation bandwidth is larger. However, a passive balun is lossy and bulky because of the transformer. Therefore, a passive balun is difficult to implement on integrated chips.
On the other hand, a conventional active balun is illustrated in FIG. 2. The active balun of FIG. 2 includes a differential amplifier 10 and a load 20. The differential amplifier 10 includes two transistors M1, M2 whose sources are connected to ground. The first transistor M1 has an input port at its gate, and the gate of the second transistor M2 is connected to a capacitor C1 whose opposite end is grounded for AC grounding. The differential output signals OUT1, OUT2 are obtained from the drains of transistors M1, M2. The loads 21, 22 are high impedance networks. The active balun has comparably large balance errors owing to its asymmetry. Furthermore, as the single-ended input signal increases, transistor M1 goes to saturation prior to transistor M2, and the leakage signal from transistor M1 enters transistor M2, and finally the large common-mode signals are amplified. Therefore, as the input signal increases, the balance errors also increase for a conventional active balun.